Aluminum die casting requires dies having both high strength and excellent toughness, the latter attribute equating generally to ductility. As is well known these attributes often tend to be offsetting in that high strength, generally with accompanying high hardness, is usually accompanied with a decrease in ductility, and vice versa. To obtain these two characteristics in the same steel therefore taxes the ingenuity of the steel producer to the limit, especially in view of the continued and increasing popularity of aluminum die casting. While zinc and magnesium die casting are also large industries, the provision of dies for these two uses are not as demanding as in the aluminum die casting industry since, of the three cast metals, aluminum is cast at the highest temperature, which may be in the region of 1200.degree. F., and is very much more reactive at its casting temperature than either magnesium or zinc, the latter of which is usually cast at about 700.degree. F. Accordingly attention has focused in recent years on developing steels and dies suitable for aluminum die casting; indeed, the commercial pressure has been so great that steel manufacturers and aluminum die casters have collaborated to establish standards to ensure that acceptable performance can be consistently obtained. Such standards, including NADCA Recommended Procedures (for) H-13 Tool Steel, published 1997, North American Die Casting Association, Rosemont, Ill., U.S.A., are very useful in introducing a degree of standards and standardization to the industry. However, only minimum acceptance standards have been promulgated and a wide area of improvement remains available for achieving near maximum performance out of the inherent maximum capabilities of the metals and available processing parameters.
In this connection the steel of choice for aluminum die casting is an AISI alloy, namely H-13, whose composition, as set out in ASTM A-681 Sec. 6 (as slightly modified for the die casting industry), is as follows:
______________________________________ C .37-.42 Mn .20-.50 P .025 max S .005 max Si .80-1.20 Cr 5.00-5.50 V .80-1.20 Mo 1.20-1.75 ______________________________________
Although steels melted to this composition and processing in conformance with the above mentioned NADCA standards yield acceptable performance, said standards provide for permissible limits of microcleanliness; that is, severity levels of the Type A, B, C and D non-metallic inclusions. In addition, said standards, while requiring that the microstructure of the steel be free of excessive banding, does recognize acceptable levels of micro-banding (i.e.: microchemical segregation) in the steel.
Elimination of non-metallic inclusions is much to be preferred however because such compounds, in any amount, are undesirable since each inclusion holds the potential for being a stress raiser which could lead, eventually, to failure in service. By the same token elimination of micro-banding is much to be desired since, again, the presence of micro-banding to any significant extent holds the potential for the initiation and propagation of cracks in use. While it may be impossible to totally eliminate micro-banding (which is often referred to as alloy segregation), a distribution of the phenomena throughout the entire work piece and, further, diffusion uniformly, is greatly to be desired.
NADCA standards recognize the probability of the presence of inclusions and micro-banding but attempt to quantify limits in order to ensure good production performance. Thus, with respect to inclusions, the following permissible limits of microcleanliness have been promulgated for thin and heavy type inclusions.